CORRESPONDENCE 103 



II 



From G. W. TYRRELL, A.R.C.Sc, F.G.S. 



Dear Sir, — The intervention of the physicists and the astronomers in 

 geological problems has frequently been unfortunate for geological science, 

 and Major Marriott's irruption into the glacial problem does not seem likely 

 to effect a change in this respect. In spite of the confident title of his recent 

 paper, " The Ice-Age Question Solved," I fear that this problem will occupy 

 the patient attention of glacialists for many weary years yet. A study of 

 the most recent textbooks indicates that astronomical theories of glaciation 

 are at present greatly discredited by geologists. Pirsson and Schuchert 

 [Textbook of Geology, 1915) devote one short paragraph in small type to only 

 one of the astronomical theories (polar wandering) in their discussion of the 

 causes of glacial climates, and remark that (p. 953) " As yet there is no 

 accepted explanation of why the earth from time to time undergoes glacial 

 climates, but it is becoming clearer that they are due rather to a combination 

 of causes than to a single cause. Perhaps the greatest single factor is high 

 altitude of the continents, with great chains of new mountains (the hypso- 

 metric causes) which disturb the general direction and constitution of the air 

 currents (the atmospheric causes) and the ocean currents as well." Chamber- 

 lin and Salisbury [Geology, vol. iii, 1906, pp. 433 et seq.) devote considerably 

 more space to hypsometric than to astronomical hypotheses. 



Glacial periods appear to be associated in geological time with the relatively 

 short periods of crustal unrest, characterised by broad and high continents, 

 shrunken oceans, and climatic extremes ; which alternated with vastly 

 longer periods of crustal quiescence, characterised by relatively small, low 

 continents, wide-spreading shallow seas, and climatic equability. This is a 

 well-established geological fact : hence it is no wonder that for the explana- 

 tion of past climatic variations, of which glaciation is only one aspect, 

 geologists are turning to purely terrestrial and geological causes, especially 

 diastrophism or crustal movements. 



Major Marriott is apparently chiefly concerned to establish the hypothesis 

 of glaciation as due to change in the obliquity of the ecliptic, based on 

 Drayson's cycle of 31,756 years instead of the orthodox astronomical cycle of 

 25,868 years. This leads to the view that the last minor glaciation (not the 

 "boulder-clay" glaciation; see R. A. Marriott, Changes of Climate, pp. 17 

 et seq.) ended about 7,000 years ago and culminated about 15,000 or 16,000 

 years ago. Major Marriott cites figures by Hoist and others, based on De 

 Geer's method of counting seasonal bands in glacial and post-glacial deposits, 

 which lend some support to this view. But it would be easy to cite figures, 

 equally reliable (or unreliable), which support the orthodox cycle, or which 

 are at variance with both. 



The differences between the figures for the two cycles are insignificant 

 from the geological point of view, for geologists cannot as yet provide such 

 exactitudes from their side of the question as can the astronomers. They have 

 Huxley's mathematical mill too much in mind to place much reliance upon 

 actual time estimates. Hence, even if astronomical hypotheses of glaci- 

 ation were entertained, geologists would be not at all concerned to choose 

 between the orthodox astronomical Tweedledum and the Draysonian 

 Tweedledee. 



One difficulty with these astronomical cycles of glaciation is their regu- 

 larity. From anything to the contrary in Major Marriott's papers they may 

 be presumed to extend back, cycle on cycle, into the remote past. Where do 

 they begin, and why ? The American evidence, according to Chamberlin 

 and Salisbury, is strongly in favour of the view that not only were the separate 

 glaciations of the Pleistocene epoch of different lengths and intensities, but 

 that the intervals between them were also variable. Is the succession of the 



